The present invention relates generally to methods for fabricating components for gas turbine engines, such as aircraft jet engines, and more specifically to a novel thermomechanical method for fabricating net shape airfoils from single crystal sheet stock.
Superalloys are metal alloys developed to withstand severe mechanical stresses at very high temperatures. Superalloys are generally nickel-based and achieve their very high strength primarily from the mechanical reinforcement effect of a relatively large volume of a gamma prime (.gamma.') precipitate phase of nickel, and other elements such as titanium and aluminum, coherently interspersed within a more continuous matrix of a gamma (.gamma.) phase. The gamma prime phase contributes to high strength primarily by resisting dislocation movement. Unfortunately, as the volume portion of gamma prime increases, the increasing resistance to dislocations makes the superalloy harder and harder to thermomechanically work, as in forging or rolling. The highest strength superalloys with very high volume fractions of gamma prime are essentially unworkable and must be cast and then machined to a final net shape.
Superalloys are made even stronger by eliminating grain boundries where slippage and breaks can occur. Single crystal superalloys are commonly used to eliminate grain boundaries and achieve the highest possible high temperature creep strength, which is especially valuable for use in aircraft jet engine rotor blades and other airfoils. Single crystal airfoils are even more difficult to work than polycrystalline superalloys, so that they are most often cast to oversize near net shapes and then machined to their final net shapes. The cost of machining and from material loss can contribute to the high cost of such aircraft engine components.
The prior art includes methods for working polycrystalline superalloys to increase their high temperature strength. Such methods include working the polycrystalline superalloy at high temperatures after annealing and/or aging steps. The prior art also includes working single crystal superalloys to increase their hardness and strength at intermediate temperatures.
Single crystal superalloys should be simpler to work than polycrystalline superalloys because they lack the complication of grain boundary strengthening additives necessary to provide high temperature strength to the polycrystalline superalloys. Unfortunately, the worked single crystal superalloys are very susceptible to recrystallization from the high temperatures that can occur during working and use. Without the presence of additives to inhibit movement along grain boundaries, the recrystallized, formerly single crystal, superalloy components have a greatly increased potential for failure. Thus, in the practical manufacture of single crystal nickel superalloys, thermomechanical working has not been considered an option.
Thus it is seen that there is a need for a practical thermomechanical method for producing net shape airfoils from single crystal sheet stock that eliminates the costs associated with machining parts from oversize castings.
It is, therefore, a principal object of the present invention to provide a thermomechanical method for producing net shape airfoils from single crystal sheet stock that eliminates the need for machining oversize castings while preserving the high temperature strength of the single crystal and avoiding recrystallization.
It is a feature of the present invention that it uses a method for preventing recrystallization of worked single crystal sheet stock that can be easily adapted for other uses.
It is an advantage of the present invention that its implementation is straightforward and uncomplicated, using conventional equipment.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.